Whooping cough, or pertussis, typically causes the most severe disease in young children, but in recent years, there has been increasing diagnosis of pertussis in adults in some countries, including Australia and the United States.

The bacterial infection also known as pertussis can be very serious for children under the age of 12 months. The biggest outbreak is currently in Washington State, where there were more than 3,000 cases through July 14. NBC's Robert Bazell reports.

In the new study, Bette Liu, of the University of New South Wales in Sydney, and colleagues analyzed information from more than 263,000 adults ages 45 and older, who were followed for an average of 10 months between 2006 and 2008.

During the study period, 205 adults were diagnosed with pertussis, and 12 people were hospitalized.

Adults were just as likely to catch pertussis at any age, but those ages 65 and over were more likely to be hospitalized.

Among those with body mass indexes (BMIs) over 30, the incidence of whooping cough was 133 cases per 100,000 people, compared with 82 cases per 100,000 people for those with a BMI under 25. (People with BMIs between 20 and 25 are considered normal weight; those with BMIs over 30 are considered obese.)

Those with asthma and those taking medications were 60 percent more likely to have pertussis compared with those without asthma, or those not taking medications.

Diagnosing pertussis in adults can be difficult because the disease's symptoms such as coughing and fever, occur commonly in other conditions. It is likely that more people were infected than diagnosed, the researchers said.

It is not clear whether people who are obese, have asthma or take medications are truly more predisposed to pertussis, or whether these people are simply more likely to go to the doctor to be diagnosed, the researchers noted.

Obesity is known to predispose adults to other respiratory infections, such as pneumonia and the flu, and so it is possible it also increases the risk of contracting pertussis, the researchers said.

The study was published online July 17 in the journal Clinical Infectious Diseases.

NBC's Dr. Nancy Snyderman urges parents to immunize their children, and says older kids and adults should get the pertussis booster shots. 'If we all get vaccinated, we can protect everyone,' she said.